1. Field of the Invention
The present invention relates to a dot printer, and more particularly to a dot printer capable of reducing a density difference among printed characters and printing characters at a high speed.
2. Description of the Prior Art
In a prior art thermal dot printer which prints characters by a head having a plurality of heat generating resistive elements, one for each dot, the thermal head comprises, for example, seven heat generating resistive elements arranged in a line. When a character is to be printed by such a thermal head, a predetermined number of heat generating resistive elements generate heat depending on the particular character to be printed. As the number of heat generating resistive elements simultaneously energized changes, the ratio of the internal resistance of a power supply such as a dry cell which supplies power to the heat generating resistive elements, to the resistance of the plurality of heat generating resistive elements changes. As a result, the voltage applied to the heat generating resistive elements changes and the print density varies.
The print density in such a thermal dot printer is determined by the heat generated by the dots. In order to attain a uniform print density, it is necessary to keep the heat per dot of the thermal head constant. ##EQU1## where R(.OMEGA.) is a resistance, V(volt) is a voltage applied to the respective dot and t(ms) is a heating time.
Assuming that the resistance per dot is 11.OMEGA. and the heat per dot is 2.1 mJ, the voltage applied to the head and the heating time have a relation shown in FIG. 1. When a manganese cell or an alkali manganese cell is used as a power supply, the following phenomena are observed.
(1) The saturation voltage of the driver used changes as the electromotive force drops.
(2) The internal resistance of the dry cell increases as the electromotive force drops.
(3) The voltage applied to the head changes as the load changes due to a change in the number of dots which simultaneously generate heat.
(4) The electromotive force of the dry cell is recovered when it is left unloaded. The recovery voltage under no load is larger as the electromotive force drops.
Because of those factors, the print density varies. Since the ratio of the internal resistance of the dry cell to the impedance of the printer changes as the number of dots which simultaneously generate heat changes, the voltage applied to the head during the print operation significantly changes. An example of a relation between the number of heat generating dots and the resistance of the printer is shown below.
______________________________________ Printer Printer Operation Resistance R ______________________________________ voltage check (4-phase excitation) 7.5 .OMEGA. 1-dot printing (2-phase excitation + 1 dot) 6.3 .OMEGA. 2-dot printing (2-phase excitation + 2 dots) 4.0 .OMEGA. 3-dot printing (2-phase excitation + 3 dots) 2.9 .OMEGA. 4-dot printing (2-phase excitation + 4 dots) 2.3 .OMEGA. 5-dot printing (2-phase excitation + 5 dots) 1.9 .OMEGA. 6-dot printing (2-phase excitation + 6 dots) 1.6 .OMEGA. 7-dot printing (2-phase excitation + 7 dots) 1.4 .OMEGA. Remarks 4-phase pulse motor winding resistance: 30 .OMEGA./phase 1 .times. 7 thermal head 11 .OMEGA./dot ______________________________________
FIG. 2 illustrates the change of net voltage applied to the head as the load impedance changes due to a change of the number of dots which simultaneously generate heat. As seen from the above table and FIG. 2, the voltage applied to the head significantly changes depending on the number of dots which simultaneously generate heat. A character font of a thermal printer incorporated in an electronic desk-top calculator is limited to numberals and symbols for an office equipment or a special purpose equipment, unlike a terminal printer. FIG. 3 shows an example of a character font used in a thermal printer of a desk-top calculator. When the dots of one vertical line of the character font shown in FIG. 3 are simultaneously energized to print the character in a conventional control method, the characters "1", "4" and ".sqroot." have unclear portions as shown in FIG. 4. In those characters, since the number of dots which are simultaneously energized is large, the net voltage applied to the head drops to 3 volts when a dry cell having an electromotive force of 6 volts is used and the number of simultaneously energized dots is seven. As a result, the print density is low. The same problem is also encountered in a wire dot printer.
In the prior art, in order to prevent the voltage drop, the heat generating resistive elements are energized sequentially one dot at a time to reduce the number of simultaneously energized dots. FIG. 5 is a time chart illustrating a timing relation between heat generating elements DT1-DT7 and energization time in printing the numeral "1" in the prior art control system. When the prior art control system (time division control system) shown in the time chart of FIG. 5 is applied to all characters, a long time is required to print one line of characters. That is, since the energization times of the respective heat generating elements are delayed by a predetermined time period from the previous one so that the respective dots are sequentially energized in a partially overlapped manner, a long print time is required to print all characters in the line. For example, when seven dots are energized, a time period of tH shown in FIG. 5 would be required in the conventional control method but a time period of 3 tH is required when three dots are "simultaneously" energized. In such a time division control method, since the dots are sequentially energized even for those characters which can be printed in the conventional control method, a long print time is required.
It has been proposed to print those characters which have a large number of simultaneously energized dots such as characters "1", "4" and ".sqroot." in the time division control method and print those characters which have a small number of simultaneously energized dots in the conventional control method so that the print density variation is reduced and the print time is shortened. However, it is ineffective and uneconomical to switch the control method for each of 20-30 characters.